Heat treatment of liquid food products, such as milk, is a commonly occurring industrial process today. By heating the product, extended shelf-life will be obtained by the extermination of those microorganisms which are to be found in the product. In the sterilisation of the food product, it is heated to temperatures exceeding 100° C. In order rapidly to heat to such elevated temperatures, steam is employed. The heating may take place either directly or indirectly. In indirect heating, use is made of different types of heat exchangers. In direct heating, steam is added directly to the product.
There are two types of direct heating of a liquid product, injection and infusion. In injection, steam is injected into the product in a closed system. Infusion implies that the product is finely divided and caused to pass through a space filled with steam. In both cases, the supplied steam rapidly and efficiently heats up the product to the desired temperature and the product is then kept at this temperature during a given predetermined interval of time. The supplied steam must thereafter be removed from the product in order to avoid diluting it. This normally takes place by evaporative cooling, so-called flashcooling, in a vacuum chamber. During the process, the steam is released and condensed at the same time as the product is cooled down to the temperature it had before the heat treatment.
The evaporative cooling usually takes place in that the steamed product is fed, under pressure, into a vacuum chamber. When the product enters into the vacuum chamber, the liquid boils, the steam is released and rises upwards in the chamber while the product accumulates in the lower region of the chamber. Thus cooled, the product may be tapped off from the lower region of the chamber. The steam which leaves the product together with incondensable gases is to be condensed in order for it to be able to be run off to an outlet. The condensation may be put into effect either in that the steam and the gases are led into a further vacuum chamber where the steam is cooled by being showered with cold water, or that the steam is condensed in some form of water-cooled plate condenser or tube condenser. The plate or tube condenser may be integrated in the first vacuum chamber or alternatively be placed outside it.
The majority of the apparatuses in existence today for condensing the steam are relatively expensive to manufacture since, in the first case, an extra vacuum chamber is required, or alternatively some form of condenser is needed. For the conventional method of condensing the steam, a considerable quantity of coolant water is moreover consumed, and this water should be of good quality so as to avoid limestone furring and corrosion on plates or tubes in the condenser.
Swedish Patent Specification SE 514 560 discloses an apparatus for evaporative cooling which only utilises one vacuum chamber. The vacuum chamber is divided into two concentrically placed spaces which are open upwards towards the upper end wall of the chamber. The steamed product enters into the one space, and in the second space the released steam is showered with coolant water from a closed circulation circuit. Nor does this apparatus require any expensive and complicated condensers. However, one drawback inherent in this apparatus is that there is a risk that the coolant water which is employed for condensing the steam may splash over to the second space and thereby dilute the product, or even worse run the risk of infecting the sterile food product. By showering with coolant liquid from above in the one space, there is also created a cold surface against the product space which may result in the steam in the product being condensed too early and that a part of the steam thereby accompanies the product out from the plant.